1. Technical Field of the Invention
This invention generally relates to satellite telephony applications. More particularly, the invention relates to a method for increasing efficiency in satellite telephony systems so as to enhance power savings at the satellite during multiple channels per carrier (MCPC) satellite transmissions.
2. Description of Related Art
FIG. 1 illustrates a block diagram of a satellite telephony system 100 used for conventionally-known voice activated transmissions such as MCPC, VAD, VOX and VP transmissions. The system 100 comprises an air or space-borne satellite 110 which is in communication with a plurality of telephone subscribers or users 120, via satellite links 125 to/from antennas 130 and radio units 135 corresponding to each of the users 120. The satellite 110 also has transceiver or radio circuitry, and both the satellite 110 and the radio units 135 are able to perform transmission, reception, modulation and demodulation functions for bursts of packets (frame data) transmitted therebetween.
In conventional satellite telephony applications, a known power saving method is based on the fact that a telephone user speaks, on average, less than 50% of the time. During the silence time between speaking, the system 100 ceases transmission of background noise. The individual radio units 135 locally generate the background noise, so that background noise data is not transferred using the satellite link 125. This results in a substantial saving of the satellite's 110 power and energy, which is important since the satellite 110 is generally the limiting component in the satellite telephony system 100. This is because the satellite 110 has a limited amount of power which is driven from sun cells, power which is required to be used for all transmissions through the satellite 110.
This current method is quite efficient for a single telephone call, but degrades in situations where there is more then one call. To understand this more clearly, current single and multiple user efficiency models, as applied to satellite telephony applications, are briefly explained below.
(a) Single User Efficiency.
In telephony systems, the theoretical efficiency for a single user is about 50%. A typical user listens and does not speak more than 50% of the conversation. For satellite telephony systems, the actual efficiency depends on the voice activity implementation, as well as the user's speech pattern.
Transmit power can be an acute problem for satellite 110. One way to save satellite power is to reduce the power consumption of the remote systems on the ground that are taxing the satellite 110. This may be accomplished using a voice activated detection (hereinafter VAD) algorithm. For example, let T be a silence time (also called in-active time) in which a user does not speak. Within the satellite telephony system 100, implementation of a VAD algorithm stops RF transmission during the silence time, and resumes RF transmission as soon as the user starts to speak. If τ1 is the transmission time that is required in order to start a transmission burst (preamble time), and if τ2 is the transmission time needed to stop a transmission burst (postamble time), then in a conventional VAD algorithm, the efficiency (E) of a voice detect (VD) operation is given by:                                           E            =                                          (                                  T                  -                                      τ                    ⁢                    1                                    -                                      τ                    ⁢                    2                                                  )                            T                                ,                                                    if                ⁢                                                                   ⁢                T                            >                              (                                                      τ                    ⁢                    1                                    +                                      τ                    ⁢                    2                                                  )                                      ;            or                          ⁢                                   ⁢                                  ⁢                              E            =            0                    ,                                    if              ⁢                                                           ⁢              T                        ≤                                          (                                                      τ                    ⁢                    1                                    +                                      τ                    ⁢                    2                                                  )                            .                                                          (        1        )            
Accordingly, use of the current VAD algorithm causes additional delay to the system. For example, assume g, called the gap, is the minimal delay between the end of one burst and the beginning of a second burst. The gap represents the time required for transmission, reception, modulation and demodulation operations to be performed between bursts. The delay (D) that is added to the satellite telephony system 100 is given by the following expressions:D=τ1, if T>(τ2+g); or D=τ1+τ2+g+T, if T≦(τ2+g).  (2) 
Both sets of equations (1) and (2) therefore suggest that VAD benefits are increased as T increases, and that the satellite telephony system 100 is better off avoiding the performing of VAD operations (note the greater delay D) if T is too small.
Current VAD algorithms also use what is called “hung-over time” to increase the average length of T. Hung-over time is a time that the VAD algorithm will wait (a considerable length of non-active time) before declaring silence. These algorithms that utilize hung-over time assume that the distribution function of T for an average telephone represents two assumptions regarding silence time. The first is that silence during speech bursts last only sub-seconds. An example of silence during speech burst might be silence between sentences. The second assumption is that silence time is present when the speaker listens to the other side's conversation. This silence interval may last seconds up to even several minutes. Although in reality there are more short (sub seconds) silence intervals than long silence intervals, most of the short silence intervals last only milliseconds.
The use of hung-over time therefore prevents the VAD algorithm from declaring silence for the first η milliseconds, (usually between 100-200 milliseconds). Thus, short pauses during speech burst will not trigger termination of the transmission burst. Since this increases the probability that only a burst having a long silence period will be terminated, it follows that VAD efficiency E will increase, with a corresponding decrease in the average delay D.
(b) Multiple-Users efficiency.
The multiple user model is different from the single user model. The combined silence time is the product of an AND operation of all the active users silence time. Accordingly, the characteristics of this combined silence time is different from the single user case. Namely, the silence probability PSingle=0.5 (50%) for a single user changes to PMultiple=0.5N, where N is the number of users (channels) in the system. Moreover, in the multiple user model, the long silence intervals present when a speaker is listening break into segments of shorter silence periods-this reduces the efficiency of the VAD algorithm even further.
For an exemplary case where there are four channels (users), the probability of silence is PMultiple=0.54 or about 6% or less. Thus, for multiple transmission of multiple channels (MCPC transmissions), use of the current VAD algorithm described above does not save any significant power. Particularly, the cost of stopping transmission is too high, since the necessity of maintaining the broadcast outweighs any power savings (negligible in the multiple user case) to the satellite. Moreover, no thought has been given regarding a way to save the satellite's transmit power to begin with, even though it is readily recognized that the satellite's transmit power is a limiting feature in satellite telephony applications.
Therefore, what is desired is a method which addresses the aforementioned limitations in current VAD algorithms regarding aggregated or multiple channels, which provides a power saving algorithm resulting in greater average power efficiency, so as to conserve the satellite's power consumption.